1. Field of the Invention
This invention relates to a heat insulating combustion chamber for a ceramic engine and the like, and a method of producing the same.
2. Description of the Prior Art
A conventional heat insulating structure for the wall of a combustion chamber in an engine is disclosed in, for example, Japanese Utility Model Laid-Open No. 58824/1985 filed by the applicant of the present invention. The heat insulating structure for the wall of the combustion chamber in an engine is such that a wall member composed of a porous ceramic material having a thickness of not more than 2.0 mm and a porosity of not less than 80%, and a layer of coating of a ceramic material having a thickness of not more than 0.1 mm and formed on the outer surface of the wall member, or a plate of a metal, such as stainless steel bonded to the outer surface of the wall member form an insulating wall, this heat insulating wall being provided on the inner surface of a cylinder head, the top end surface of a piston and the inner circumferential surface of a cylinder liner, which contact a combustion gas in the engine. In such a heat insulating structure for the wall of a combustion chamber for an engine, it is very difficult to combine an outer layer of coating of a ceramic material or a plate of a metal, such as stainless steel with the surface of the wall member of a ceramic material. If the porosity of the wall member is increased, the strength thereof decreases, though the heat insulating performance thereof is improved, so that it becomes more difficult to combine such an outer layer or metal plate with the surface of the wall member. If the wall member is made thicker so as to combine such a layer of coating with the wall member of a ceramic material more easily, the thermal capacity of the wall member increases. Consequently, the temperature in the combustion chamber becomes constantly high, and the suction efficiency in a suction stroke of the engine decreases.
Therefore, there has been a problem of how to construct a heat insulating combustion chamber which is capable of securing its heat insulating functions, and which has a combustion gas-exposed wall surface with the smallest possible thermal capacity and a sufficiently high strength.
The techniques for forming a layer of coating of a ceramic material by chemical vapor deposition have already been disclosed in publications. The chemical vapor deposition is put into practice in various technical fields by utilizing the permeation characteristics thereof. It is utilized for sealing bores, bonding materials and parts, plating inner surfaces of minute bores and narrowly spaced materials and parts, and forming heat resisting, wear resisting and corrosion resisting protective films, decorative films and films of a functional substance having electrical and optical characteristics. A fluidized chemical vapor deposition apparatus uses a starting substances for chemical vapor deposition a coating reagent containing as a main component a substance to be applied to a substrate, and a gas source, such as a carrier gas and a reactive gas mixed with the vapor of the reagent and sending the plating vapor to the surface of the substrate in a reaction chamber. The coating reagent used consists mainly of a volatile metal or a halide, and the carrier gas and reactive gas a simple-substance gas composed of a hydrogen gas in most cases and nitrogen and argon in some cases and a hydrocarbon gas (refer to "Ceramic Coating Techniques" published on May 25, 1984 (date of issue) by the Sogo Gijutsu Center K.K. (publishing company)).
In order to bond or chemically combine a ceramic material, such as Si.sub.3 N.sub.4 and Si C to or with the wall of a combustion chamber, the chemical vapor deposition is conveniently used. The bonding of, for example, Si.sub.3 N.sub.4 to the wall of a combustion chamber is effected by mixing gases, such as Si Cl.sub.4, NH.sub.3 and H.sub.2 together, and subjecting the resultant mixed gas to a reaction in a chemical vapor deposition furnace, i.e. a high-temperature furnace.